This error is usually as a result of installing 2 or more of one manufacturer (e.g huawei, zte, etc) modem type on one PC. It would eventually result to clashing of drivers, ports and sometimes dashboard. See the procedure below for how to fix.
How to fix the "modem is already in use" Error
- Open Control panel > Programs and Features
- Uninstall every single modem software in the list (branded and unbranded)
- Restart the PC
- Connect the modem and reinstall its software
- Ensure the APN is set correctly